Passivation becomes important when discussing corrosion resistance of metals that make up manufacturing equipment. Passivation is a process in which a passive layer forms on the SS surface, occurring naturally in the presence of oxygen when the surface has been cleaned of exogenous matter [1-3]. The passive layer on the SS surface becomes the primary means of protection to prevent corrosion. Stainless steel can corrode when the chromium-to-iron ratio has been significantly reduced, resulting in the oxidation and subsequent release of iron oxides that form deposits on surfaces.

Rouge is the commonly used term for the visible corrosion product of SS and it can be composed of several forms of iron oxides, ferric oxide being the predominant form [8-11]. Rouging is typically found in water generation systems, process tanks, and pipeline systems that are routinely exposed to corrosive solutions. Rouge seems to be a common problem, Regulatory Agencies such as the United States (US) Food and Drug Administration (FDA) has cited in at least one warning letter that corrosion is unacceptable in direct-contact pharmaceutical systems [4]. The reasoning is that rouge on product contact surfaces can create an environment for process residues and microbes to tenaciously adhere to the rouged area and become more difficult to clean and sanitize [5-7]. Residues and microbes might also reside within the rouge layers, where the routine cleaner and sanitizer may not be able to penetrate.

Considering the risk associated with rouged surfaces, manufacturers would benefit from focusing more attention on treatments that prevent rouge from happening. Some companies take a reactive approach and wait until rouge has been detected or has impacted production before taking corrective action. Process attributes such as elevated temperature, extreme pH solutions, or surface damage (like poor quality welding, etc) can corrode SS surfaces [8-11]. If a process or surface condition is already known to lead to corrosion at some point during the life of the equipment, then an effort should be made to investigate and prevent that corrosion from occurring.

Unlike preventive maintenance, which is done with the goal to mitigate the cause of the potential problem or undesirable situation, corrective maintenance is done to correct a problem or fault once it has been detected. For example, with a stainless steel preventive maintenance procedure, an operator knows exactly what needs to be done at a pre-defined schedule. On the contrary, in the case of a corrective maintenance, the critical parameters and overall procedure would depend on what is found. Corrective maintenance often requires evaluation of the severity of the rouge problem before any treatment can be recommended. This evaluation is referred to as a risk assessment, and includes a review of the potential impact to the patient, product, personnel and equipment [12]. Once the rouge is formed, there may be unknown variables associated with it and correcting the problem typically takes much longer than a preventive task. Many published references about SS corrosion are focused on corrective maintenance [3-15].

There is no globally accepted test to guarantee that a SS surface has been adequately passivated. Generally, when SS equipment has been exposed to a passivation treatment some documentation should be generated describing the passivation procedure with emphasis on the critical parameters. A test matrix has been suggested to be used as a guide for acceptance criteria and to confirm that a surface is passivated [3]. In general, the overall scope of these test methods is to either verify that the surface has been passivated by removal of exogenous matter (e.g. free iron or hydrophobic films) or by direct measurement of the quality of the passive layer. Table 1 summarizes the advantages and disadvantages of common passivation test methods [2-3]. Representative coupons of similar material and finish to the production equipment can be used to validate a passivation protocol. Passivation treatments are very dependent on the chromium content, surface finish and machinability characteristics of the grades in each family of SS. The chemical treatment should help restore the inert oxide layer in a consistent, faster forming passive layer than found naturally. Passivation treatment using citric acid blends and phosphoric acid blends are highly effective for a large number of stainless steel families.

Laboratory testing has been successful in determining critical cleaning parameters for passivation of stainless steel surfaces. A formulated phosphoric/citric acid detergent and a formulated citric/oxalic acid detergent were used to treat 316L stainless steel coupons in the passive layer evaluation experiments.

The following procedure was employed for the experiment:

A large biotech manufacturer consistently observed rouge in select 316L stainless steel buffer preparation and storage tanks. Based on solubility profiles of the buffer components, purified water should be effective at removing the buffer residue. However, persistent rouge impacted the use of the equipment. Upon visual inspection of the tanks, a third party service provider specializing in stainless steel cleaning and maintenance was required. This process resulted in increased maintenance costs and decreased production time due to equipment downtime. The biotech manufacturer investigated their cleaning and maintenance procedure to develop a scientific, risk-based approach to equipment preventive maintenance.

The biotech manufacturer investigated their cleaning and maintenance procedure to develop a scientific, risk-based approach to equipment preventive maintenance.

The objectives for the investigation were as follows:

(1) Confirm cleaning of the buffer residue
(2) Confirm condition of exposure that resulted in loss of passive layer
(3) Confirm passivation treatment that protects surface from buffer exposure

The testing of condition 1 was performed by applying 1 ml of buffer onto a pre-cleaned, 304 stainless steel coupon with a 2B finish. The treated coupon was dried at ambient temperature for a pre-specified dirty hold time. The coupon was then cleaned with deionized water for 5 minutes at 65C or 80C. The coupon was determined to be cleaned if it met visually clean, water-break free and no weight change (gravimetric testing). The testing of conditions 2 and 3 were performed as noted above with the following results.

Results for the investigation were as follows:
De-ionized water was effective in cleaning surfaces with dried buffer A (60mM Sodium Phosphate, 2 M Chloride, pH 5.6) after 5 minutes at either 65C or 80C.

Coupons coated with the Buffer A and dried overnight resulted in a non-passive surface. Concentrations of 10-15% v/v formulated citric/oxalic acid detergent at times of 20-60 minutes at 80C or at times of 40-60 minutes at 65C were effective at passivating the buffer exposed surfaces and maintained a passive condition for at least a 24-hour buffer exposure.

Surfaces maintained their passive state under the following buffer exposure time:
• Up to 48 hours after passivation with 10% v/v formulated citric/oxalic acid detergent

at 65°C for 40 minutes.
• Up to 72 hours after passivation with 15% v/v formulated citric/oxalic acid detergent

at 65°C for 60 minutes.
• Up to 24 hours after passivation with 10 % v/v formulated citric/oxalic acid detergent

at 80°C for 20 minutes.
Additional testing was performed with 1M chloride buffer solution, and an increased concentration and duration of passivation treatment with a formulated citric/oxalic acid detergent to extend wet buffer storage time. The following conditions were evaluated.

• Decreased chloride concentration in Buffer A preparation solution from 2M to 1M.
• Evaluate passivation condition of 15% v/v formulated citric/oxalic acid detergent at 80oCfor 60 or 90 minutes prior to buffer exposure.
• Extend duration of buffer hold time to greater than 72 hrs post passivation condition.The decrease in chloride concentration in the Buffer A from 2M to 1M chloride as well as the increase in 15% v/v citric/oxalic acid detergent exposure from 60 to 90 minutes extended the buffer storage condition exposure from 96 to 120 hrs.

Water is effective at cleaning Buffer A with 2M or 1M chloride. However, dirty hold time and solution storage time can affect the passive properties of the stainless steel surfaces when evaluated using the electrical pen test and cyclic polarization (not reported). The citric/oxalic acid detergent passivation conditions (time, temperature and concentration) can affect the passive layer and the duration of buffer storage time. A passivation treatment of at least 5% v/v of a formulated citric/oxalic acid blend at 80oC for 90 minutes provided a passive surface when evaluated with the electrical pen, cyclic polarization and XPS/ESCA. The passivation of the stainless steel coupon with 15% v/v citric/oxalic acid detergent maintained a passive surface using the electrical pen test for greater than 96 hours when exposed to the 1M chloride buffer.

This information enabled the manufacturer to place timers on select buffer storage and product hold tanks to inform the operators when passivation cycle is needed.

The second case study is similar to the first noted above. A multinational plasma fractionation product manufacturer was observing micro pitting and rouge in select 316L stainless steel buffer preparation tanks. The buffers included 0.1 M sodium chloride, 0.15 M sodium chloride, 1 M sodium chloride, 3 M sodium chloride, 20mM sodium acetate, 2.1 M NaCAP, pH 5 acetate, 1 M acetic acid and 1M acetic acid with 1 M sodium chloride.

The objectives for the investigation were as follows:

(1) Confirm cleaning of the buffer residue.
(2) Confirm condition of exposure that resulted in loss of passive layer.
(3) Confirm passivation treatment that protects surface from buffer exposure.

The testing of conditions 1-3 were performed as noted above with the following results.

Results for the investigation were as follows:
All the buffer solutions when air dried onto a 304 stainless coupon with a 2B finish were effectively cleaned using deionized water within 5 minutes at ambient temperature in a low mixing agitated immersion bath.

The pH 5, acetate and 1M acetic acid with 1 M sodium chloride buffers exposed to 316L stainless steel passivated coupons affected the passive layer between 72 and 96 hours as determined by electrical pen test and confirmed using copper sulfate and salt spray cabinet testing.

Washed and rinsed 316L stainless steel coupons, passivated with a 10% v/v formulated phosphoric/citric acid detergent at 80oC for 40 minutes were then rinsed with water and air-dried at ambient temperature for 1 hour before testing. The coupons were exposed to the pH 5 acetate or 1M acetic acid with 1 M sodium chloride buffers and every 3 to 4 days the coupons were removed from the buffer, rinsed with water and washed with a low concentration phosphoric/citric acid detergent (0.5% v/v, 80oC for 10 minutes). The surfaces remained passive throughout the 31-day test period.

Water is effective at cleaning the buffers evaluated. However, the 1M acetic acid with 1 M sodium chloride dirty hold time and solution storage time can affect the passive properties of the stainless steel surfaces when evaluated using the electrical pen test, copper sulfate and salt spray testing. An acid wash treatment of at least 0.5% v/v formulated phosphoric/citric acid detergent every 3 to 4 days at 80oC for 10 minutes provided a passive surface over 31 days while the non acid washed controls failed around 72 hours.